Investigators began looking at a possible connection to what employees had consumed after an initial sweep of the building showed no indication of carbon monoxide or other exposure to hazardous chemicals.

"We're chasing down other possible leads, and food poisoning is one of them," Scott said.

A 911 call about a person at the complex having breathing problems was made at about 9:58 a.m., emergency management and fire officials said. As the morning continued, more people in the complex's south building reported similar trouble and were transported by ambulance to area hospitals. One person drove to the hospital.

Emergency management officers responded in droves, with 20 emergency vehicles parked at the scene at one point.

Standard tests for carbon monoxide, explosives, hydrogen sulfide, low oxygen and other hazards turned up no link to the illness.

As emergency crews searched for clues, evacuated workers milled around a covered parking area and the side of the building, many without coats. Some sat in the warmth of their cars.

About 12:30 p.m., a man in a suit told the crowd they could go into the building's auditorium.

A Hopkins official later said the institutions wanted to make sure whatever caused the illness couldn't be transmitted to others.

"We don't know if it is something they will carry away with them," said Jonathan Links, Hopkins chief risk officer.

Sickened employees were being wheeled by gurney into ambulances as late as 1:30 p.m.

Shortly after, officials began allowing those who had been evacuated to go home, with the directive that they call 911 if they begin to feel ill, Scott said.

Officials also swept the north building of the complex, and found nothing. Employees in that building, who had also been prevented from leaving, also were eventually allowed to go home.

Officials do not believe the illness is life-threatening.

"We will make sure we check every avenue and rule everything out," said Lawrence Goldberg, battalion chief in the city fire department.

In an email to employees late Tuesday, Hopkins said both of the complex's buildings would be closed Wednesday as city and state health authorities continued to investigate.

The complex, at 3910 Keswick Road, is a mixed-use facility that includes nearly 415,000 square feet of space. The north and south sides of the building house about 1,050 employees who do administrative functions for the Johns Hopkins University and The Johns Hopkins Hospital and Health System.

A nonprofit entity named FSK Land Corp., owned jointly by the Johns Hopkins University and the Johns Hopkins Health System, completed its $15 million purchase of the former Zurich Insurance Co. property in 2010, and began investing millions more in renovations.

Hopkins officials first announced in December 2009 their intent to purchase the 11-acre property, which also has a 1,088-car parking garage. Zurich had vacated the property the previous October, after consolidating in Owings Mills.